The present invention relates to apparatus and method for thermomechanical pulping of lignocellulosic material, particularly wood chips.
In recent decades, the quality of mechanical pulp produced by thermomechanical pulping (TMP) techniques has been improving, but the rising cost of energy for these energy-intensive techniques imposes even greater incentives for energy efficiency while maintaining quality. The present inventor has already advanced the state of the art as embodied in the Andritz RTS, RT Pressafiner, and RT Fibration, process technologies. He discovered an operating window by which feed material is preheated for a very short residence time at high temperature and pressure, then refined at such high temperature and pressure between opposed discs rotating at high speed. (U.S. Pat. No. 5,776,305). A further improvement was directed to pretreating the feed chips before preheating, by conditioning in a pressurized steam environment and compressing the conditioned chips in the pressurized steam environment. (PCT/US98/14718). Yet another improvement is disclosed in International Application PCT/US2003/022057, where the feed chips discharged from the pretreatment step, are fiberized without fibrillation, for example with a low intensity refiner, before delivery to a high intensity refiner.
The underlying principle in the progression of the foregoing developments has been to distinguish and handle in distinct equipment, the axial fiber separation and fiberization of the chip material, from the fibrillation of the fibers to produce pulp. The former steps are performed in dedicated equipment upstream of the refiner, using low energy consumption that matches the relatively low degree of working and fiber separation, while the high energy consuming refiner is relieved of the energy-inefficient defibering function and can devote all the energy more efficiently to the fibrillation function. This is necessary since the fibrillation function requires even more energy than defibering (also known as defibration).
These developments did indeed improve energy efficiency, especially in systems that employ high-speed discs (i.e., above 1500 rpm for double disc and above 1800 rpm for single disc refiners). However, especially for systems that did not employ high-speed refiners, the long-term energy efficiency was offset to some extent in the short term by the need for more costly or more space-occupying equipment upstream of the primary refiner.